1. Field of the Invention
The present invention relates to apparatus for pressure testing oil or gas well equipment, and more particularly to test plugs of a type used to test Christmas trees and blowout preventers mounted on wellheads.
2. Description of the Prior Art
Blowout preventers are used on wellheads during oil and gas well drilling operations to prevent uncontrolled escape of crude oil and/or natural gas from the well which could occur when relatively high pressure is encountered in the well. The blowout preventer also serves to retain oil and/or gas within the well once it has been drilled, and Christmas trees contain valves which are used in testing the well and in controlling flow from the well. Test plugs are employed to seal off a space adjacent to the blowout preventers and the Christmas tree so that fluid under pressure can be used to pressure test these devices.
Conventional test plugs are adapted to be fitted to the end of a drill pipe string so that they may be inserted into the bore of the wellhead and removed from that bore by respectively lowering and raising the drill string. Those test plugs are landed and seated upon a relatively wide annular shoulder or other similar abutment within the bore of the wellhead and include a sealing ring which is adapted to engage the bore of the wellhead to effect a seal between the plug and the bore. Since the annular shoulder must be wide enough to support the plug when it is subject to relatively high test pressures, the shoulder constitutes a very desirable limitation on the size of the well tools or other equipment that can be passed through the wellhead.
For example, one such prior art test plug is described in U.S. Pat. No. 4,018,276 issued Apr. 19, 1977 to R. E. Bode. The Bode test plug is landed on a relatively wide annular shoulder of a seal assembly that is positioned inside the bore of the wellhead, and the plug remains on this broad shoulder throughout the testing procedure. A disadvantage of this apparatus is that the seal assembly shoulder significantly reduces the bore diameter inside the wellhead, and thus decreases the cross-sectional area through which well tools, etc., can be passed, and also limits the size of the production tubing that can be lowered through the wellhead. If the width of the annular shoulder is reduced, high pressure above the plug can cause it to be forced off the shoulder and move downwardly into a lower portion of the wellhead where it could become stuck and thus be difficult to retrieve.